The data was quietly moved around on Target's network before it was sent to a US server, then to Russia

The stolen credit card numbers of millions of Target shoppers took an international trip -- to Russia.

A peek inside the malicious software that infected Target's POS (point-of-sale) terminals is revealing more detail about the methods of the attackers as security researchers investigate one of the most devastating data breaches in history.

Findings from two security companies show the attackers breached Target's network and stayed undetected for more than two weeks.

"The intrusion operators displayed innovation and a high degree of skill in orchestrating the various components of the activity," according to a Jan. 14 report from iSight Partners, a Dallas-based information security company.

Over two weeks, the malware collected 11GB of data from Target's POS terminals, said Aviv Raff, CTO of the security company Seculert, in an interview via instant message on Thursday. Seculert analyzed a sample of the malware, which is circulating among security researchers.

The data was first quietly moved to another server on Target's network, according to a writeup on Seculert's blog. It was then transmitted in chunks to a U.S.-based server that the attackers had hijacked, Raff said.

Logs from that compromised server show the data was moved again to a server based in Russia starting on Dec. 2. Raff said it's difficult to say if the attackers are based in Russia.

"No one knows who is really behind this," he said.

ISight is working with the U.S. Secret Service to look into the Target breach, which compromised payment card and personal details of up to 110 million people between Nov. 27 and Dec. 15, 2013, the busiest shopping time of the year.

A U.S. Department of Homeland Security spokesman said Thursday that a separate, private report with input from iSight and government agencies on the Target compromise could not be publicly released.

Target has not revealed how intruders breached its network but said that its POS terminals were infected with malware.

In its Jan. 14 analysis, iSight wrote that the "Trojan.POSRAM" malware collected unencrypted payment card information just after it was swiped at Target and while it sat in a POS terminal's memory. The type of malware it used is known as a RAM scraper.

The code of "Trojan.POSRAM" bears a strong resemblance to "BlackPOS," another type of POS malware, iSight wrote. BlackPOS was being used by cyberattackers as far back as March 2013.

At the time of its discovery, Trojan.POSRAM "had a zero percent antivirus detection rate, which means that fully updated antivirus engines on fully patched computers could not identify the software as malicious," iSight said.

Small code changes are often made to malware to make it undetectable to security products, which appears to have been done in this case.

Although Trojan.POSRAM and BlackPOS are similar, the Target malware contains a new attack method that evades forensic detection and conceals data transfers, making it hard to detect, iSight wrote on its website.

Target's problems point to the difficulties of defending large, Internet-connected networks, said Levi Gundert, a former Secret Service agent and now a technical lead for threat research, analysis and communications at Cisco.

"It's literally impossible to prevent unauthorized access to the network," Gundert said in a phone interview.